The present invention relates to a circuit arrangement for coupling alternating voltage signals in the audio frequency range between a low noise audio frequency source and a zero ohm amplifier, wherein the circuit arrangement has a highly resistant input which is more resistant by at least one power of ten than the source impedance of the audio frequency source and the amplification of the circuit arrangement is so low that no overshooting will happen at the maximum occurring voltage of the voltage signals received at the input of the circuit arrangement.
Such a circuit arrangement is disclosed in German Pat. No. 2,925,049. It can be used to particular advantage if a microphone, for example a capacitor microphone or a dynamic microphone, preferably with a subsequently connected transformer which transforms the microphone output voltage up somewhat, is to be connected to a so-called zero ohm or zero node amplifier. The present invention is particularly useful if the zero ohm amplifier is a controlled, particularly a voltage controlled, amplifier, i.e., a so-called VCA.
German Pat. No. 2,925,049 refers to a specific VCA configuration, but the present invention can also be used for other VCA's in which generally, as in the prior art reference, an operational amplifier with nonlinear feedback is provided in the input stage.
The principles of the prior art arrangement are illustrated in FIG. 1. An audio frequency source T, including a microphone M and a subsequently connected transformer U, is connected to a gain controlled, zero ohm amplifier N which is essentially composed of an operational amplifier OP with a controllable (variable) feedback resistance V. The zero ohm amplifier has a very low input resistance of approximately 0 ohm which ordinarily would present too much of a load current for the audio frequency source T. Therefore, an impedance converter W is connected therebetween, whose positive output resistance is significantly less than its input resistance and whose voltage amplification has been selected to be so small that no overshooting will happen at its maximum occurring input voltage. Between impedance converter W and zero ohm amplifier N there is also a series resistor L whose resistance value is relatively low so that it does not contribute much noise. However, series resistor L cannot be completely omitted because otherwise the output of impedance converter W would be stressed too much by the low input resistance of zero ohm amplifier N.
Additionally, series resistor L is also responsible for the noise contribution made by the operational amplifier OP itself. This noise contribution can be considered to originate from a noise voltage source directly at the inverting input of operational amplifier OP. The noise voltage of the noise voltage source is amplified by the operational amplifier with V and L connected as shown, wherein the so-called internal gain of zero ohm amplifier N increases as the resistance value L decreases.
With the configuration shown in FIG. 1, the impedance converter W and the relatively small series resistor L still contribute such large amounts of noise, due to the increase of the internal gain of zero ohm amplifier N, that the latter would have to be designed to be correspondingly lower in noise if stricter requirements for a signal with even less noise at the output of the zero ohm amplifier are to be met. However, it has been found to be very difficult and expensive to design a variable gain zero ohm amplifier so that it is correspondingly low in noise.